Field of the Invention
The invention relates to a method of operating a three phase primary winding structure of a system for inductive power transfer and a primary unit of said system for inductive power transfer.
Description of Related Art
Vehicles, in particular electric vehicles and/or a track-bound vehicle and/or a road automobile, can be operated by electric energy which is transferred by means of an inductive power transfer.
The inductive power transfer is usually performed using a primary unit which generates an alternating electromagnetic field and a secondary unit which comprises a receiving device for receiving said electromagnetic field.
The primary unit and the secondary unit can e.g. each comprise a set of three-phase windings. A set of windings of the primary unit can be installed on the ground (primary winding structure) and can be fed by a wayside power converter (WPC). A set of windings of the secondary unit (secondary winding structure) is installed on the vehicle. For example, the secondary winding structure can be attached underneath the vehicle, in the case of trams under some of its wagons.
Furthermore, such a vehicle can comprise a rectifier adapted to convert an alternating current (AC) to a direct current (DC). The DC can be used to charge a traction battery or to operate an electric machine. A rectifier converts the AC provided by the receiving device into the DC.
The first and the secondary side, in particular the primary and the secondary winding structure, can be part of a high frequency transformer to transfer electric energy to the vehicle. This transfer can be done in a static state (when there is no movement of the vehicle) and in a dynamic state (when the vehicle moves).
To provide an efficient inductive power transfer, it is desirable to have a tuned inductive power transfer system. This means that a resonant frequency of primary and secondary winding structures which depends on the capacitive and inductive elements of these winding structures matches a predetermined operating frequency. In a tuned system, the maximal power transfer efficiency can be reached.
The aforementioned resonant frequency depends on the mutual coupling between the primary and secondary winding structure. As the mutual coupling depends on an alignment of the primary winding structure relative to the secondary winding structure, the alignment will have an impact on the power transfer efficiency. In particular in the case of a misalignment, the power transfer efficiency decreases.
Moreover, if power needs to be transferred to an automobile inductively, it is desirable to have small dimensions of a primary unit. In this case, however, even small misalignments will result in a significant decrease of the power transfer efficiency.